1. Field of the Invention
The present invention relates to an implantable heart stimulating device with which it is possible to stimulate both the ventricles of a heart, i.e. a bi-ventricular pacer.
The invention also relates to a system including such a device and to a method of, in a human or animal being, performing a capture threshold search.
2. Description of the Prior Art
Several different implantable devices for stimulating a heart are known. The devices are normally able to sense the electrical activity of the heart, Some implantable devices are able to deliver stimulation pulses to and/or sense the right atrium (in some case even the left atrium) and also to deliver stimulation pulses to and sense both the left and right ventricle.
Devices that are able to deliver stimulation pulses to both the left and right ventricle can be called bi-ventricular pacers. Such devices can be used to treat patients who suffer from different severe cardiac problems, e.g. patients suffering from congestive heart failure (CHF). CHF is defined generally as the inability of the heart to deliver a sufficient amount of blood to the body. CHF can have different causes. It can for example be caused by a left bundle branch block (LBBB) or a right bundle branch block (RBBB). By using bi-ventricular pacing, the contraction of the ventricles can be controlled in order to improve the ability of the heart to pump blood. The stimulation pulses to the two ventricles can be delivered simultaneously but it is also known that the stimulation pulses to the two ventricles are delivered with a short time delay between them in order to optimize the pumping performance of the heart.
U.S. Pat. No. 5,720,768 describes different possible electrode positions in order to stimulate or sense the different chambers of the heart.
U.S. Pat. No. 6,070,100 describes that electrodes may be positioned in both the left and the right atrium as well as in the left and the right ventricles.
In connection with implantable pacers, it is known to detect the capture of the heart, i.e. to detect whether the heart actually reacts to a delivered stimulation pulse. If the heart is not captured, it is possible to arrange the pacer to deliver a back-up pulse with a higher pulse energy than the first pulse. It is also possible to increase the pulse energy in future stimulation pulses if capture is not detected. In order to save battery it is important that the stimulation pulses are not delivered with an unnecessarily high energy. In order to determine a suitable pulse energy, it is known to perform an automatic threshold/capture search. By varying the energy of the stimulation pulses, and by detecting whether capture occurs, it is thus possible to find a threshold value for the stimulation pulse energy. Based on the threshold value, a suitable stimulation pulse energy can be determined.
The detection of capture involves several problems. Different signals from the heart or generated by the pacemaker may interfere with each other, which may make the detection of capture difficult. The evoked response that it is intended to detect may thus be hidden because of other electrical phenomena. It is particularly difficult to detect capture in a bi-ventricular pacer, since in such a pacer there are more delivered and detected signals which may interfere with each other.
A phenomenon in this technical field is “fusion”. A fusion may occur when an intrinsic depolarization of the heart takes place simultaneously, or at least almost simultaneously, with a stimulation pulse from the heart stimulating device. Fusion should be avoided when performing a threshold/capture search. In order to avoid such fusion, it is known to temporarily, during such a threshold/capture search, shorten the AV-delay and the PV-delay. Typically, these delays are shortened such that they are set at a predetermined fixed value (e.g. 50 ms and 25 ms, respectively) during such a search.
U.S. Pat. No. 6,498,950 describes a device and method for performing automatic capture/threshold determination in a mono-ventricular pacer. The patent describes a method and a device that instead of using the mentioned fixed AV-delay and the PV-delay during a threshold/capture search, use delays adapted to the particular patient. According to this patent, the device periodically measures the AR/PR conduction times and tabulate and/or otherwise process this data. When an automatic capture/threshold determination occurs, this measured conduction data, which corresponds to the particular patient, is used to adjust the AV/PV delays while minimizing patient discomfort and adverse hemodynamic effects.